Plant lipids have a variety of industrial and nutritional uses and are central to plant membrane function and climatic adaptation. These lipids represent a vast array of chemical structures, and these structures determine the physiological and industrial properties of the lipid. Many of these structures result either directly or indirectly from metabolic processes that alter the degree of unsaturation of the lipid. Different metabolic regimes in different plants produce these altered lipids, and either domestication of exotic plant species or modification of agronomically adapted species is usually required to produce economically large amounts of the desired lipid.
Plant lipids find their major use as edible oils in the form of triacylglycerols. The specific performance and health attributes of edible oils are determined largely by their fatty acid composition. Most vegetable oils derived from commercial plant varieties are composed primarily of palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2) and linolenic (18:3) acids. Palmitic and stearic acids are, respectively, 16- and 18-carbon-long, saturated fatty acids. Oleic, linoleic, and linolenic acids are 18-carbon-long, unsaturated fatty acids containing one, two, and three double bonds, respectively. Oleic acid is referred to as a mono-unsaturated fatty acid, while linoleic and linolenic acids are referred to as poly-unsaturated fatty acids. The relative amounts of saturated and unsaturated fatty acids in commonly used, edible vegetable oils are summarized below (Table 1):
TABLE 1Percentages of Saturated and Unsaturated FattyAcids in the Oils of Selected Oil CropsMono-Poly-SaturatedunsaturatedunsaturatedCanola6%58%36%Soybean15%24%61%Corn13%25%62%Peanut18%48%34%Safflower9%13%78%Sunflower9%41%51%Cotton30%19%51%
Corn oil is comprised primarily of even-numbered carbon chain fatty acids. The distribution of fatty acids in typical corn oil is approximately 12% palmitic acid (16:0), 2% stearic acid (18:0), 25% oleic acid (18:1), 60% linoleic acid (18:2), and 1% linolenic acid (18:3). Palmitic and stearic acids are referred to as saturated fatty acids because their carbon chains contains only single bonds and the carbon chain is “saturated” with hydrogen atoms. Oleic, linoleic, and linolenic acids contain one, two, and three double bonds respectively, and are referred to as unsaturated fatty acids. Fatty acids in corn oil nearly always occur esterified to the hydroxyl groups of glycerol, thus forming triglycerides. Approximately 99% of refined corn oil is made up of triglycerides (“Corn Oil”, Corn Refiners Association, Inc., 1001 Connecticut Ave., N.W., Washington, D.C. 20036, 1986, 24 pp.).
Many recent research efforts have examined the role that saturated and unsaturated fatty acids play in reducing the risk of coronary heart disease. In the past, it was believed that mono-unsaturates, in contrast to saturates and poly-unsaturates, had no effect on serum cholesterol and coronary heart disease risk. Several recent human clinical studies suggest that diets high in mono-unsaturated fat and low in saturated fat may reduce the “bad” (low-density lipoprotein) cholesterol while maintaining the “good” (high-density lipoprotein) cholesterol (Mattson et al. (1985) Journal of Lipid Research 26:194–202).
A vegetable oil low in total saturates and high in mono-unsaturates would provide significant health benefits to consumers as well as economic benefits to oil processors. As an example, canola oil is considered a very healthy oil. However, in use, the high level of poly-unsaturated fatty acids in canola oil renders the oil unstable, easily oxidized, and susceptible to development of disagreeable odors and flavors (Gailliard (1980) in The Biochemistry of Plants Vol. 4, pp. 85–116, Stumpf, P. K., ed., Academic Press, New York). The levels of poly-unsaturates may be reduced by hydrogenation, but the expense of this process and the concomitant production of nutritionally questionable trans isomers of the remaining unsaturated fatty acids reduces the overall desirability of the hydrogenated oil (Mensink et al. (1990) N. Eng. J. Med. N323: 439–445).
When exposed to air, unsaturated fatty acids are subject to oxidation which causes the oil to have a rancid odor. Oxidation is accelerated by high temperatures, such as in frying conditions. The rate of oxidation is enhanced in the cases of oils containing greater degrees of unsaturation. Thus, linoleic acid with two double bonds is more unstable than oleic acid which has only one double bond. Oxidation reduces the shelf life of products containing corn oil because of that oil's high proportion of linoleic acid. Corn oil and products containing corn oil are often packaged under nitrogen in special packaging materials such as plastic or laminated foil, or are stored under refrigeration to extend their shelf life. These extra measures to reduce oxidation and subsequent rancidity add considerable cost to products containing corn oil.
Another measure to reduce the effects of oxidation on corn oil is to chemically hydrogenate the oil. This commercially important process by which hydrogen is added to double bonds of unsaturated fatty acids changes the physical properties of the oil and extends the shelf life of products containing corn oil. Hydrogenated vegetable oils are used to make margarine, salad dressings, cooking oils, and shortenings, for example. Approximately half a billion pounds, or roughly 40–50% of corn oil produced in the U.S. is used for cooking and for salad oils (Fitch, B., (1985) JAOCS, Vol. 62, no. 11, pp. 1524–31). Production of a more stable oil by genetic means would clearly have value by reducing or eliminating the time and input costs of chemical hydrogenation.
In addition to the economic factors associated with chemical hydrogenation of corn oil, there are human health factors that favor the production of a natural high oleic oil. During the hydrogenation process, double bonds in fatty acids are completely hydrogenated or are converted from the cis configuration to the trans configuration. Cis double bonds cause a fatty acid molecule to “bend,” which impairs crystallization and keeps the oil liquid at room temperature. During hydrogenation, cis bonds are straightened into the trans configuration, causing the oil to harden at room temperature. Recent studies on the effect of dietary trans fatty acids on cholesterol levels show that the trans isomer of oleic acid raises blood cholesterol levels at least as much as saturated fatty acids, which have been know for some time to raise cholesterol in humans (Mensink, R. P. and B. K. Katan, (1990) N. Engl. J. Med., 323:439–45). Furthermore, these studies show that the undesirable low density lipoprotein level increases and the desirable high density lipoprotein level decreases in response to diets high in trans fatty acids. Large amounts of trans fatty acids are found in margarines, shortenings, and oils used for frying; the most abundant trans fatty acid in the human diet is the trans isomer of oleic acid, elaidic acid.
While oils with low levels of saturated fatty acids are desirable from the standpoint of providing a healthy diet, fats that are solid at room temperature are required in some foods because of their functional properties. Such applications include the production of non-dairy margarines and spreads, and various applications in confections and in baking. Many animal and dairy fats provide the necessary physical properties, but they also contain both cholesterol and cholesterogenic medium-chain fatty acids. An ideal triglyceride for solid fat applications should contain a predominance of the very high melting, long chain fatty acid, stearic acid, and a balance of mono-unsaturated fatty acid with very little polyunsaturated fat. Natural plant solid fat fractions typically have a triacylglyceride structure with saturated fatty acids occupying the sn-1 and sn-3 positions of the triglycerides and an unsaturated fatty acid at the sn-2 position. This overall fatty acid composition and triglyceride structure confers an optimal solid fat crystal structure and a maximum melting point with minimal saturated fatty acid content.
The natural fat prototype for this high melting temperature vegetable fat is cocoa butter. More than 2 billion pounds of cocoa butter, the most expensive commodity edible oil, are produced worldwide. The U.S. imports several hundred million dollars worth of cocoa butter annually. High and volatile prices together with the uncertain supply of cocoa butter have encouraged the development of cocoa butter substitutes. The fatty acid composition of cocoa butter is 26% palmitic, 34% stearic, 35% oleic and 3% linoleic acids. About 72% of cocoa ‘butter's triglycerides have the structure in which saturated fatty acids occupy positions 1 and 3 and oleic acid occupies position 2. Cocoa ‘butter's unique fatty acid composition and distribution on the triglyceride molecule confer on it properties eminently suitable for confectionery end-uses: it is brittle below 27° C. and depending on its crystalline state, melts sharply at 25°–30° C. or 35°–36° C. Consequently, it is hard and non-greasy at ordinary temperatures and melts very sharply in the mouth. It is also extremely resistant to rancidity. For these reasons, producing corn oil with increased levels of stearic acid, especially in corn lines containing higher-than-normal levels of palmitic acid, and reduced levels of unsaturated fatty acids is expected to produce a cocoa butter substitute in corn. This will provide additional value to oil and food processors as well as reduce the foreign import of certain tropical oils.
The human diet could also be improved by reducing saturated fat intake. Much of the saturated fat in the human diet comes from meat products. Poultry and swine diets often contain animal fat, which is high in saturated fatty acids, as an energy source. Non-ruminant animals such as these are very susceptible to tissue fatty acid alteration through dietary modification (M. F. Miller, et al. (1990) J. Anim. Sci., 68:1624–31). A large portion of animal feed rations is made up of corn, which typically contains only about 4% oil. By replacing some or all of the supplemental animal fat in a feed ration with the oil present in high oil corn varieties, which contain up to 10% oil, it will be possible to produce meat products having a lower content of saturated fats. Feeding trials in which swine were fed diets high in oleic acid show that the amount of oleic acid deposited in adipose tissue can be raised substantially without adversely influencing the quality of the meat (M. F. Miller, et al.; L. C. St. John et al. (1987) J. Anim. Sci., 64:1441–47). The degree of saturation of the fatty acids comprising an oil determines whether it is liquid or solid. In these studies, the animal diets high in oleic acid led to meat quality that was acceptable to the meat processing industry because of the low level of polyunsaturated fatty acids.
Only recently have serious efforts been made to improve the quality of corn oil through plant breeding, especially following mutagenesis, and a wide range of fatty acid composition has been discovered in experimental lines. These findings (as well as those with other oilcrops) suggest that the fatty acid composition of corn oil can be significantly modified without affecting the agronomic performance of a corn plant.
There are serious limitations to using mutagenesis to alter fatty acid composition. It is unlikely to discover mutations that a) result in a dominant (“gain-of-function”) phenotype, b) are in genes that are essential for plant growth, and c) are in an enzyme that is not rate-limiting and that is encoded by more than one gene. Even when some of the desired mutations are available in mutant corn lines, their introgression into elite lines by traditional breeding techniques will be slow and expensive, since the desired oil compositions in corn are most likely to involve several recessive genes.
Recent molecular and cellular biology techniques offer the potential for overcoming some of the limitations of the mutagenesis approach, including the need for extensive breeding. Some of the particularly useful technologies are seed-specific expression of foreign genes in transgenic plants [see Goldberg et al. (1989) Cell 56:149–160], and the use of antisense RNA to inhibit plant target genes in a dominant and tissue-specific manner [see van der Krol et al. (1988) Gene 72:45–50]. Other advances include the transfer of foreign genes into elite commercial varieties of commercial oilcrops, such as soybean [Chee et al.                Plant Physiol. 91:1212–1218; Christou et al. (1989) Proc. Natl. Acad. Sci. U.S.A. 86:7500–7504; Hinchee et al. (1988) Bio/Technology 6:915–922; EPO publication 0 301 749 A2], rapeseed [De Block et al. (1989) Plant Physiol. 91:694–701], and sunflower [Everett et al. (1987) Bio/Technology 5:1201–1204], and the use of genes as restriction fragment length polymorphism (RFLP) markers in a breeding program, which makes introgression of recessive traits into elite lines rapid and less expensive [Tanksley et al. (1991) Bio/Technology 7:257–264]. However, application of each of these technologies requires identification and isolation of commercially-important genes.        
WO 91/13972, published Sep. 19, 1991, describes desaturase enzymes relevant to fatty acid synthesis in plants, especially delta-9 desaturases.
U.S. Pat. No. 5,443,974, issued to Hitz et al. on Aug. 22, 1995, describes the preparation and use of nucleic acid fragments encoding soybean seed stearoyl-ACP desaturase enzymes or its precursor to modify plant oil composition.
WO 94/11516, published May 26, 1994, describes genes for microsomal delta-12 desaturases and related enzymes from plants. The cloning of a corn (Zea mays) cDNA encoding seed microsomal delta-12 fatty acid desaturase is described. The discussion of that citation is hereby incorporated by reference.
Oil biosynthesis in plants has been fairly well-studied [see Harwood (1989) in Critical Reviews in Plant Sciences Vol. 8(1): 1–43]. The biosynthesis of palmitic, stearic and oleic acids occur in the plastids by the interplay of three key enzymes of the “ACP track”: palmitoyl-ACP elongase, stearoyl-ACP desaturase and acyl-ACP thioesterase. Stearoyl-ACP desaturase introduces the first double bond on stearoyl-ACP to form oleoyl-ACP. It is pivotal in determining the degree of unsaturation in vegetable oils. Because of its key position in fatty acid biosynthesis it is expected to be an important regulatory step. While the ‘enzyme's natural substrate is stearoyl-ACP, it has been shown that it can, like its counterpart in yeast and mammalian cells, desaturate stearoyl-CoA, albeit poorly [McKeon et al. (1982) J. Biol. Chem. 257:12141–12147]. The fatty acids synthesized in the plastid are exported as acyl-CoA to the cytoplasm. At least three different glycerol acylating enzymes (glycerol-3-P acyltransferase, 1-acyl-glycerol-3-P acyltransferase and diacylglycerol acyltransferase) incorporate the acyl moieties from the cytoplasm into triglycerides during oil biosynthesis. These acyltransferases show a strong, but not absolute, preference for incorporating saturated fatty acids at the sn-1 and sn-3 positions and monounsaturated fatty acid at the sn-2 of the triglyceride. Thus, altering the fatty acid composition of the acyl pool will drive a corresponding change in the fatty acid composition of the oil due to the effescts of mass action. Furthermore, there is experimental evidence that, because of this specificity, and given the correct composition of fatty acids, plants can produce oils suitable as cocoa butter substitutes [Bafor et al. (1990) JAOCS 67:217–225].
Based on the above discussion, one approach to altering the levels of stearic and oleic acids in vegetable oils is by altering their levels in the cytoplasmic acyl-CoA pool used for oil biosynthesis. There are two ways of doing this genetically. One of these ways is to alter the biosynthesis of stearic and oleic acids in the plastid by modulating the levels of stearoyl-ACP desaturase in seeds through either overexpression or antisense inhibition of its gene. Another is converting stearoyl-CoA to oleoyl-CoA in the cytoplasm through the expression of the stearoyl-ACP desaturase in the cytoplasm.
In order to use antisense or sense inhibition of stearoyl-ACP desaturase in the seed, it is essential to isolate the gene(s) or cDNA(s) encoding the target enzyme(s) in the seed, since either of these mechanisms of inhibition requires a high-degree of complementarity between the antisense RNA (see Stam et al. (1997) Annals of Botany 79:3–12) and the target gene. Such high levels of sequence complementarity or identity is not expected in stearoyl-ACP desaturase genes from heterologous species.
The purification and nucleotide sequences of mammalian microsomal stearoyl-CoA desaturases have been published [Thiede et al. (1986) J. Biol. Chem. 262:13230–13235; Ntambi et al. (1988) J. Biol. Chem. 263:17291–17300 and Kaestner et al. (1989) J. Biol. Chem. 264:14755–14761]. However, the plant enzyme differs from them in being soluble, in utilizing a different electron donor, and in its substrate-specificities. The purification and the nucleotide sequences for animal enzymes do not teach how to purify a plant enzyme or isolate a plant gene. The purification of stearoyl-ACP desaturase was reported from safflower seeds [McKeon et al. (1982) J. Biol. Chem. 257:12141–12147] and from soybean (U.S. Pat. No. 5,443,974).
The rat liver stearoyl-CoA desaturase protein has been expressed in E. coli [Strittmatter et al. (1988) J. Biol. Chem. 263:2532–2535] but, as mentioned above, its substrate specificity and electron donors are quite distinct from that of the plant.
Plant stearoyl-ACP desaturase cDNAs have been cloned from numerous species including safflower [Thompson et al. (1991) Proc. Natl. Acad. Sci. 88:2578], castor [Shanklin and Somerville (1991) Proc. Natl. Acad. Sci. 88:2510–2514], and cucumber [Shanklin et al. (1991) Plant Physiol. 97:467–468]. Kutzon et al. [(1992) Proc. Natl. Acad. Sci. 89:2624–2648] have reported that rapeseed stearoyl-ACP desaturase when expressed in Brassica rapa and B. napa in an antisense orientation can result in increase in 18:0 level in transgenic seeds.
Manipulation of stearate levels has been described (Knutzon, D. S. et al., (1992) Proc. Natl. Acad. Sci. USA 89(7): 2624–2628). It is possible to elevate the level of stearate seed oils by underexpression of stearoyl-ACP desaturase, the enzyme responsible for introducing the first double bond into 18 carbon fatty acids in plants. Seeds from both B. campestris and B. napus plants produced by antisense expression of a cDNA encoding the B. campestris stearoyl-ACP desaturase using a seed specific promoter region produced oils high in stearic acid, but also contained elevated levels of linolenic acid (18:3) when compared to unmodified plants from the same species. Elevated levels of stearic acid have been obtained in soybean by a similar underexpression of stearoyl-ACP desaturase (U.S. Pat. No. 5,443,974) and in canola by overexpression of an acyl-ACP thioesterase (U.S. Pat. No. 5,530,186). Mutation breeding has also produced soybean lines with elevated levels of stearic acid in their seed oils (Graef, G. L. et al., (1985) JAOCS 62:773–775; Hammond, E. G. and W. R. Fehr, (1983) Crop Sci. 23:192–193).
Poly-unsaturated fatty acids contribute to the low melting point of liquid vegetable oils. In high saturate oils their presence is a detriment in that they decrease melting point, and therefore even higher levels of undesirable saturated fatty acid are required to achieve a plastic fat at room temperature. Additionally, when used in baking and confectionery applications, high levels of poly-unsaturates leads to oxidative instability as described above for liquid oils. Thus for maximum utility a high saturate fat produced in corn should contain saturated fatty acids, mono-unsaturated fatty acid and as little poly-unsaturated fatty acid as possible. Gene combinations discovered in this invention provide novel fatty acid profiles in corn which meet these criteria. Other combinations result in a lipid profile in which the oleic acid content is not less than 60% of the total oil content. Many of these combinations also utilize a novel corn oleosin promoter or an intron/exon region from the shrunken 1 gene, or both an oleosin promoter and an intron/exon region from the shrunken 1 gene.
Lipid reserves in corn seeds are synthesized and stored primarily in a specialized tissue of the embryo called the scutellum. These lipid reserves constitute up to 50% of the dry weight of the embryo at seed maturity. As in all seeds, the storage lipid in corn seeds is packaged into simple organelles called oil bodies. These small spherical organelles consist of a triacylglycerol core surrounded by a single layer of phospholipids embedded with proteins termed oleosins (Huang (1985) Modern Methods of Plant Analysis 1: 175–214; Stymme and Stobart (1987) The Biochemistry of Plants 10: 175–214; Yatsue and Jacks (1972) Plant Physiol. 49: 937–943; and Gurr (1980) The Biochemistry of Plants 4: 205–248).
At least two classes of oleosin isoforms have been identified in diverse species of plants (Tzen et al. (1990) Plant Physiol. 94: 1282–1289). These two classes are arbitrarily named as high (H) and low (L) molecular weight isoforms within a particular species. Members of one isoform from diverse species are understood to be structurally related based on demonstrations of shared immunochemical properties and possession of significant amino acid sequence identity, and they are clearly distinct from members of the other isoform (Hatzopoulos et al. (1990) Plant Cell 2: 457–467; Lee and Huang (1994) Plant Mol. Biol. 26(6): 1981–1987; Murphy et al. (1991) Biochim. Biophys. Acta, 1088: 86–94; Qu and Huang (1990) J. Biol. Chem. 265: 2238–2243).
There are three oleosin isoforms present in corn seeds. They are found in the approximately proportional amounts of 2:1:1. These isoforms are named OLE16, OLE 17, and OLE 18, corresponding to their apparent molecular weights which range from approximately 16 kDa to 18 kDa. OLE17 and OLE18 are closely related members of the H class, whereas OLE16 is a member of the L class (Lee and Huang, 1994). The genes encoding the three oleosins have been cloned and sequenced (Qu and Huang (1990) J. Biol. Chem. 265: 2238–2243; and Huang, personal communication). The genes are expressed only in tissues within the embryo (scutellum and embryonic axis) and the aleurone layer during seed development, and are positively regulated by the hormone abscissic acid (Vance and Huang (1988) J. Biol. Chem. 263: 1476–1491; Huang (1992) Annu. Rev. Plant Physiol. Plant Mol. Biol. 43: 177–200). The oleosins are highly expressed in the embryo, representing about 5–10% of the total scutellum protein or 2–8% of the total seed proteins.
Promoters from genes that display an embryo- and aleurone-specific (“embryo/aleurone”) pattern of expression, such as the oleosin genes, would be attractive candidates for use in transgenic approaches to direct the expression of a gene encoding an oil-modifying enzyme (Qu and Huang (1990) J. Biol. Chem. 265: 2238–2243; and Huang (1992)) or other enzymes of interest for embryo-specific traits, especially in corn. Another potential candidate gene from which to isolate a corn embryo/aleurone-specific promoter is the maize globulin-1 gene (Belanger and Kriz, 1989, Plant Physiol. 91: 636–643). However, to date, there is no report that describes the expression, regulation, or use of such promoters in either transient expression assays or stably integrated transgenic corn plants.